Certain naturally occurring materials contain a hydrocarbonaceous component which upon heating will release a hydrocarbon product which is useful as a feedstock in petroleum processing. These "hydrocarbonaceous solids" such as oil shale, tar sands, coal and diatomaceous earth, may be pyrolyzed in reactor vessels having various designs. Prior to pyrolysis the solid feed usually must be reduced to a particulate material having a certain maximum particle size dependent on the type of retorting process. Unfortunately, with oil shale, which is a fissile rock, and to a lesser extent with the other materials, crushing and grinding usually yield a variety of particle sizes ranging from a fine dust to large chunks. The inability of conventional grinding and crushing operations to produce a feed of uniform particle size leads to downstream processing problems when a fluidized bed is used to pyrolyze the oil shale because such processes are able to tolerate only a limited range of particle sizes. In those processes using a recycled heat-transfer material, the solids handling problem is compounded by the increased volume of material.
The use of a partially fluidized bed to control the passage of solids through the pyrolysis zone is one means for extending the ability of the retort to handle a wide range of particle sizes. An example of this type of bed is the staged turbulent bed described in U.S. Pat. No. 4,199,432. However, such improvements still require that the amount and maximum size of non-fluidizable particles be controlled within specified limits. Since the crushing and grinding step in most oil shale processes significantly increases in cost as the maximum particle size and/or the amount of the largest particles are decreased, it is desirable to design any process to be able to handle as large a particle size as possible.
The present invention is concerned with a modified oil shale retorting process which increases the maximum particle size which can be tolerated in a retorting system using a fluidized particulate heat-transfer material.